Protection of electric circuits



Sept. 16, 1930. H. s. PETH PROTECTION OF ELECTRIC CIRCUITS Filed Sept.19, 1928 Tnvenbor: Herbert, S. Fetch, by His Abner-negfault.

Patented Sept. 16, 1930 UNITED STATES PATENT OFFICE HERBERT STANLEYFETCH, OF EDGWARE, ENGLAND, ASSIGNOR TO GENERAL ELEC- TRIO COMPANY, ACORPORATION OF NEW YORK PROTECTION OF ELECTRIC CIRCUITS Applicationfiled September 19, 1928, Serial No. 306,972, and in Great BritainOctober 19, 1927.

My invention relates to improvements in lay between the two phasesaffected and the protection of electric circuits such as thus afiordcomplete protection.- So that, in feeders, interconnectors, or sectionsthereof, transformers and the like, of the kind in 5 which a neutralpoint of the circuit is earthed. It relates not only to protectivesystems of the kind in which the current flow at one end of theprotected circuit is balanced against the current flow at the other endof the protected circuit, or in which the currents flowing in twocircuits in parallel are compared, by means of similar protectiveapparatus at each end, or on each parallel circuit such as protectivesystems of the 5 circulating current or opposed voltage type, but it isalso applicable to protective systems in which the protective apparatusoperates at a pointon a protective circuit to open the circuit inresponse to an interphase or earth relays are required to give completeprotection. y

In the protective system described in British Patent No. 188,001, one ofthe current transformers aforesaid of each protective apparatus isreplaced by a double or split-Winding transformer, and the connectionsare such that only two relays need be used with each protectiveapparatus, one responding to all interphase faults on the protectedcircuit and the other to earth faults.

An arrangement which has recently been proposed for protecting athree-phase circuit involves a transformer device provided with windingsof n, (n-H22.) and (n+2m) turns, respectively, which are energized re-Usually a protective relay is arsponsively to the currents in therespective ranged to control a circuit for the trip coil of phases ofthe protected circuit. A secona circuit breaker controlling theprotected dary winding. on this transformer device is circuit, although,if preferred, the protective then connected to a simple relay, and thisdevice may be arranged to operate a warn relay will respond either toearth faults or 5 ing signal instead of actually effecting the tointerphase faults. In this system the opening of the protected circuitunder fault number m is relatively small compared with diti 91,, forexample, in the nature of 3 to 20. The

In a simple polyphase protective system,-theory of operation of thissystem is dethe secondaries of current transformers enscribed in apamphlet issued bythe Conergizes responsively to current in theinference Internationale des Grands Reseaux dividual phases may beend-connected to- Electriques and numbered 74. gether and provided withprotective relays My invention is an improvement in, or in series witheach secondary winding. modification of, the arrangement last de- Theserelays are then responsive to interscribed, in which one simple relaymay be phase faults occurring on the protectedcirused for protecting apolyphase circuit cuit. An earth fault responsive relay may againstearth faults or interphase faults. then be connected across theend-connected Broadly speaking, it comprises the combinapoints of thesecondary windings, this relay tion with a transformer device, havingwindbei'ng energized responsively to the vector 'ings of a materiallydifferent number of sum of the currents in the protected circuit. turnsenergized responsively to currents in Now, this protective apparatus;often in-' cludes four relays for a three-phase circuit a polyphasecircuit, and to the vector sum but, in ractice, one of the interphasefault of thesecurrents, respectively, of a protective device, thewinding, of which is energized from the transformer device and which isreresponsive relays may be omitted-,as an interphase faultwill thenoperate the other resome of the individual phase conductors of general,for an w-phase polyphase circuit as I Eti- sponsive either toearth-faults or to interphase faults. An object of my invention is toeflect a considerable reduction in the number of turns of thetransformer device.

, My invention will be better understood from the following descriptionwhen considered in connection with the accompanying drawing and itsscope will lie-pointed out in the appended claims.

In the accompanying drawing,Fig, 1 illustrates diagrammatically anembodiment of my invention and Fig. 2 illustrates a modification of myinvention.

In. the embodiment of my invention shown in Fig. 1, the transformerdevice is shown as ,including a core 1 having three primary windings 2,3 and 4 of n, m, and Emturns, respectively, where m is small comparedwith n. A protective relay 5 is connected to an auxiliary or secondarwinding 6 on the core 1. This results in t e saving of 2% turnsaltogether over the arrangement abovedescribed. The relay 5, as shown,is arranged to control a circuit breaker 7 in a three phase circuitincludin conductors 8, 9, 10 by means of its trip coi 11, operatingcurrent for which may be supplied from a suitable source such as acontrol bus 12. Associated withthe three phase conductors 8, 9, 10 arecurrent transformers 13, 1d, respectively whichare assumed of likeratios and which are providedter energizing the winding??? and d ofmaterially diflerent turns. winding fl ofQm turns is connected invseries with the secondary of the current transformer 15 associatedwiththe conductor 10; the winding 3 of m turns is connected in series withthe secondary of the current trans.- former 1d associated with theconductor 9, and no winding with that of conductor 8 in the same sense.The other ends oil the windings 3 and d of m and 2m turns, are, however, end-connected to the appropriate end oi the secondary winding otthe current transformer 13 associated with conductor 8 and the free endsof the secondary windings oi all the current transformers are starconnected. The winding of nturns is then ar ranged in a residual currentcircuit connect ing the common or star point of the current transformersecondary windings with the common point of the two windings 3 and t ofm and 2m turns. 4 The winding 2 is therefore connected to be energizedin accordance with the vector sum of the currents in the phaseconductors'S, 9, 1O; -.'lhis residual current circuit may include animpedance 16.

With this arrangemenh if-the auxiliary or secondary winding 6 on thetransformer device requires a magnetornotive force of M ampere turns,for the protective device to be operated, and if r is the ratio oftransformation in the current transformers '13, 1d, 15,

the following represent the values of the fault currents in therespective phase conductors underdifi'erent conditions for whichoperation willbe eflected If we assume M to equal 400 ampere turns, andr to be 10, and if we take a equal to 200 and m equal to 30, the valuesof fault currents necessar to operate the protective device in the dierent conditions are approximately as follows: v

approximately. I

The divergency between the earth fault currents and the interphase faultcurrents on which the protective device responds may, however, beadjusted by the use of the impedance 16 above referred to. This iniedance performs its function by virtue or the fact that it imposes anadditional burden on ,the'current transformers in earth fault conditions only. As a result of this additional burdenthe ratio of theparticular current transformer on the conductor 'which is grounded willineffect decrease. Gonsequently, if a suitable impedance is added in theleakage circuit, it will so modify the ratio of the transformers underleakage fault conditions that the above figures will becomeapproximately i V (i 77." (ii 65. (iii 57. (iv 133. (v .133.

till

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current or opposed electromotive forces as will be apparent to thoseskilled in the art. In the latter case, as shown relays 19 may beconnected in series in the pilot circuit while in the former they may beconnected across normally equipotential points. This arrangement reducesthe number of pilot conductors to two at the same'time providingprotection ing connected to be energized rcsponsively to the sum of theeffects of said primary windings.

4. A protective arrangement for a three phase circuit includinga-transformer device aving a plurality of windings of n,.m and 2m turnsrespectively, where m is small relatively to n,the windings m and 2mturns being connected to be energized respectively in accordance withthe currents in two of the phase conductors of the circuit and thewinding of n turns being connected to be energized in accordance withthe residual current of the circuit, and an impedance connected inseries with the winding of n turns.

In witness whereof, I have hereunto set my hand this fourth day ofSeptember, 1928.

HERBERT STANLEY FETCH.

against both earth and interphase faults with v the minimum number ofrelays.

While I have shown and described my invention in considerable detail, Ido not desire to be limited to the exact arrangements shown, but seek tocover in the appended claims all those modifications that fall withinthe true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is

1. A protective arrangement for a three phase circuit including atransformer device having a plurality of windings of n, m and 2m turnsrespectivel where m is small relatively to n, the win 'ngs of m and 2mturns being connected to be energized respectively in accordance withthe currents in two of the (fhase conductors of the circuit and the wining of 1:. turns be connected to be energized in accordance with theresidual cur- 7 rent of the circuit.

2. A protective arrangement for a three phase circuit including atransformer device aving a plurality of windings of n, m and 2m turnsres ctivel where m is small relazievely to 1a,o:;t::dwintu'a omnd 2mturns ing conn toener res tivel in accordance with the currents in tv iof mi phase conductors of the circuit and the windmg of '1: turns beingconnected to be energized in accordance with theresidual current of thecircuit, and relay means connected to be energized in accordance withthe sum of the effects of said windings.

3. A protective arrangement for a three phase circuit including atransformer device having a plurality o primar windings of n, m and 2mturns respective y, where m is small relatively to a, the windings of mand 2m turns being connected to be energized respectivel 1n accordancewith the currents in two of t e phase conductors of the circuit and thewinding of it turns be' connected to be energized in accordancewicurrent of the circuit, and a secondary windthe residual

